Reaction motor burn times can be controlled by various approaches depending upon the nature of the fuel in the reaction motor. For example, if one or all of the fuel components of a reaction motor are in fluid form, it is possible to control the burn time merely by shutting off the flow of one of these components. If, however, the rocket motor is designed to use nonflowing fuel materials, such as is the case of a typical solid propellant rocket motor where a propellant grain of particular size and configuration is located within a combustion chamber, it is not possible to control the flow of a propellant component to the combustion chamber. In this type of rocket motor, the propellant is designed to produce a predetermined thrust-time curve but as mission requirements vary, it sometimes becomes necessary to terminate the thrust prior to the time at which the entire propellant grain has been consumed. Thus, it becomes necessary to somehow extinguish the combustion process within the combustion chamber prior to consumption of the entire grain. One approach to accomplishing this is by physically quenching the combustion process by injection of a liuqid such as water which will absorb a substantial amount of heat upon vaporization and reduce the temperature of the grain below that at which it will continue to burn. This approach, of course, requires rapid injection of relatively large amounts of fluid and necessarily requires the carrying of an amount of combustion termination fluid on board. This necessarily detracts from the available payload. Another approach which is more readily used in large solid propellant rocket motors is to depressurize the combustion chamber. As a result of the burning characteristics of many solid propellants, the combustion process is pressure sensitive and it is known that a rapid depressurization of the combustion chamber can extinguish the combustion process on the grain. Two approaches are used in accomplishing this - forward-end thrust termination and aft-end thrust termination. In the forward-end thrust termination system, a large port or a plurality of small ports are provided through the forward closure which ports are sealed by plates adapted to be released, for example, by means of explosive bolts or Primacord. When the appropriate signal is obtained, the explosive is detonated and the port covers ejected, causing rapid depressurization. The disadvantage with forward-end thrust termination is that it may interfere with the location of the payload, produce a sharp rearward or axially offset acceleration jolt, require a more complex internal geometry for the propellant in the forward end and the debris produced by the port is ejected at a place where it can cause damage to the rocket vehicle. Also, the port area is limited by the weight of propellant which must be eliminated. Thus, in some cases, the mission requirements prohibit the use of ports large enough to prevent the chamber pressure from building up to the re-ignition pressure where the motor will spontaneously re-ignite.
When aft-end thrust termination is employed, one approach is to explosively sever the nozzle assembly from the aft end of the rocket, thereby opening up a substantially larger exit port, causing rapid depressurization. This approach has the advantage of ejecting the debris in a direction in which it will not interfere with the rocket motor itself; however, the sudden release of a large volume of gas through the newly opened exit port of the rocket motor produces an extremely high forward acceleration jolt. In many cases this jolt, as well as the rearward jolt produced by forward end thrust termination, can be greater than can be tolerated by the payload. Because of this effect, it is sometimes not possible to open up a port large enough to maintain the pressure within the motor below the re-ignition pressure.
It is an object of this invention to provide for the reduction or elimination of the jolt which occurs on the opening of a thrust termination port.
It is another object of this invention to provide aft-end thrust termination in a manner which retains all the advantages of the previous aft termination systems with respect to damage from debris and at the same time reduces or eliminates the undesirable acceleration jolt.
It is another object of this invention to provide for thrust termination by depressurization which is capable of maintaining the pressure within the motor below the re-ignition pressure.
Another object of this invention is to provide a rocket motor nozzle assembly capable of providing aft-end thrust termination without an excessive acceleration jolt.